Conventionally, image forming apparatuses that perform the inkjet recording method are attracting attention because images can be recorded at high speed, images can be recorded onto so-called plain paper not requiring a special fixing method, and the noise during the recording operation is negligible. By such an inkjet recording method, an image is formed with the use of a recording liquid chamber (hereinafter, “ink liquid chamber”) and an ink jet head (hereinafter, “recording head”) provided with a nozzle that is in communication with the ink liquid chamber. Specifically, predetermined pressure is applied to the ink inside the ink chamber according to a print image, so that ink drops are jetted from the nozzle and the ink adheres onto an image forming medium (recording medium), thereby forming the image.
To achieve high-quality printing results at low cost and high printing speed with an image forming apparatus employing such an image forming method, it is important to devise a way of achieving good-quality print images with a recording head with limited resolution.
When a character image is printed as a print image formed with dots, the quality of the printed character varies significantly depending on the resolution of the print image to be printed. For example, when a character of the same size is printed at 300 dpi (dots per inch) and at 600 dpi (dots per inch), the number of dots in the printed character in the case of 600 dpi is four times as that of 300 dpi. Therefore, when printing at 600 dpi, the character can be reproduced in more detail and the printed character will obviously have better quality. Particularly, the slanted portions and the curved portions in the character will change in a stepwise manner according to the resolution (i.e., the number of dots will increase or decrease). Thus, when the character is printed at 300 dpi, these portions tend to be more noticeable as jaggies.
Patent document 1 discloses a conventional smoothing method referred to as anti-aliasing performed in an image forming apparatus employing the inkjet recording method. This is a method for reducing jaggies that appear when the resolution is low.
Furthermore, patent document 2 discloses a processing method as another method of correcting jaggies. Specifically, the jaggies appearing along the outline of a character are detected by performing pattern matching on a dot font corresponding to bitmap data of a character loaded in a font memory, with the use of plural jaggy correction patterns that have been prepared beforehand for the purpose of detecting and correcting jaggies. A matching pixel found as a result of the pattern matching is replaced with a dot having a small dot size (a dot reproduced with a reduced amount of adhering ink) to correct the jaggies.
Patent document 3 discloses a method of forming dots around a portion where jaggies have appeared among dots forming an outline portion of a character and/or a graphic figure in an image. The formed dots are made to have smaller sizes than those forming portions where jaggies have not appeared. Furthermore, the method of forming the dots is changed according to the slant of the outline portion.
Patent document 4 discloses the following method. In a dot font which is bitmap data of a character loaded in a font memory, jaggies that have appeared along the outline portion of the character are detected by performing pattern matching with the use of plural jaggy correction patterns prepared beforehand for the purpose of detecting and correcting jaggies. A matching pixel found as a result of the pattern matching is replaced with a dot having a small dot size (a dot reproduced with a reduced amount of adhering ink).    Patent Document 1: Japanese Laid-Open Utility Model Application No. H3-113452    Patent Document 2: Japanese Laid-Open Patent Application No. 2003-334938    Patent Document 3: Japanese Laid-Open Patent Application No. 2004-17546    Patent Document 4: Japanese Laid-Open Patent Application No. 2004-17552
However, with the method disclosed in patent document 1, at the portion where jaggies appear (for example, in the case of a character, at step-like irregularities along the outline), the number of dots is changed at numerous resolutions, and therefore the smoothing operation can be performed highly precisely but the process is extremely complicated and time consuming. Thus, this method is inappropriate for an image forming apparatus that is required to print with high performance.
Furthermore, when the above-described conventional jaggy correction methods disclosed in patent document 1 and 2 are applied to double-side printing, the following problem occurs. That is, in order to mitigate “show-through” (i.e., to prevent the character from being visible from the other side of the sheet) when performing double-side printing, the resolution of the print image is decreased so that the density of the printed characters is reduced. However, the jaggy correction dots corresponding to the density for performing single-side printing are combined with the print characters for double-side printing. Thus, the outline portions (corrected portions) of the printed characters become dark. As a result, each of the printed characters will appear to be fringed. This degrades the quality of the printed image, and the printed characters will be difficult to read.
With the jaggy correction methods disclosed in patent documents 3 and 4, the character that has been corrected is thinner than a regular character, and therefore the visibility of the printed character decreases. As a result, low-quality printed images are provided for users who use document data including numerous printed characters such as a business document.